The present invention relates to a paper machine including a headbox, a wire part in which water is drained from a paper web, a press section arranged afer the wire section in the running direction of the paper machine for pressing water out of the web, a dryer section arranged after the press section, a coating section arranged after the dryer section, an after dryer arranged after the coating section, a calender section arranged after the after dryer and a reel up arranged after the calender section. The present invention also relates to a method of making paper in a paper machine of the type described above.
Paper grades manufactured by means of prior-art methods and paper machines do not meet all present-day or future needs in respect of paper grades. One important need is glossy porous paper, which is well suited for colour-powder based printing. This need arises, among other things, from the fact that, along with increasing speeds and improved printing quality of colour copiers, in colour-powder based printing, colour copiers have started competing with sheet-offset printing and, on the other hand, this need also arises from the fact that, in black/white printing, digital processing of image and laser technology have improved the quality of the picture to the level of offset printing. One problem in printing with a colour copier is inadequate gloss of the paper to be used. Coated paper is not directly suitable for use in sheet-feed colour copiers, but one of the problems is a so-called glass-sheet effect between two sheets, i.e. the sheets tend to adhere to each other. In friction-based feeders in colour copiers, coated papers do not operate at all, and also in vacuumbased feeders they operate poorly. Thus, in colour-powder printing, such a coated or compacted paper is needed as does not adhere to the colour powder fixing unit and in whose use said problem of glass-sheet effect does not occur. It has also been a problem that humidity may remain in the interior of compacted paper when the paper is dried from both sides.
Thus, in actual fact, there is no method or paper machine suited for the manufacture of papers of this type, and of the paper grades currently available, usually a supercalendered fine paper is used, but it is a problem of this paper grade that it is not sufficiently glossy.
As is known from the prior art, said paper grades are manufactured in paper machines in which the headbox is, for example, a hydraulic headbox, for example SymFlo(trademark) or some other conventional headbox of the same type. The wire part that is used is a fourdrinier wire or a hybrid wire part, for example SymFormer(trademark), in which there are a fourdrinier wire and an upper-wire unit. Also, gap formers have been used in the manufacture of paper grades of different types. From the prior art, a method is also known for application of additives, fillers and chemicals as layers. This method is described in the patent EP 0 651 092. As a press section, presses of many types are used, usually roll presses. From the prior art, different extended-nip press solutions are also known, which have, however, not been applied to the manufacture of papers used for colour-powder based printing. The dryer sections have been made of conventional dryer sections which make use of single-wire or twin-wire draw and in which the drying takes place primarily as cylinder drying. Impingement drying is known from a number of different patent publications, but industrial applications are not in operation as yet. On the contrary, some use has been made of air drying arranged by means of the principle of infra drying or airborne-web drying. As size presses, size presses of many different types are used, for example tub size presses or solutions of the type of the applicant""s SymSizer. The calender has, as a rule, been a soft calender with one or two nips and combinations formed out of them. Also, supercalendering has been used for the manufacture of the final product. The reel-up has been a suitable reel-up. Paper machines of the types described above and component units of said machines have been described, for example, in the following published patent applications and patents: FI 75,377, FI 83,540, FI 98,540, U.S. Pat. No. 4,075,056, EP 0 770 727, FI 98,387, FI 901967, and FI 924960. By means of these machines, a paper grade well suited for colour-powder based printing has, however, not been achieved, owing to the above problems.
The prior art does not know a suitable paper machine or an applicable method for manufacture of paper that has, first, copying paper properties and good gloss and suitable porosity for colour-powder printing. Further, among traditional properties of copying paper, important properties are, among other things, properties of electric charge and resistivity and dimensional stability. It is, however, expected that the need of paper grades of this type will increase in the near future, so that a method and a paper machine are needed for manufacture of such papers.
Thus, the object of the present invention is to provide a method and a paper machine by whose means it is possible to manufacture especially a paper of this type with suitable gloss and suitable porosity.
In accordance with the invention, the paper web is formed as layers in the Z-direction so that the desired distributions of additives and fillers are obtained in the different layers in the Z-direction in the web. The web is calendered in at least one calendering nip, which maintains or at least substantially retains the porosity of the web preceding the calendering. Favourably, the web is calendered in a shoe calender which comprises an extended calendering nip. Successive calendering operations can be carried out in the same calendering device or in separate successive devices.
In accordance with the invention, the headbox and the wire part of the paper machine have been formed so that the desired composition of layers is obtained for the paper in the Z-direction. The calender is a calender device that maintains or at least substantially retains the porosity of the web preceding the calendering, for example a device provided with an extended calendering nip or a device provided with an extensible calendering nip and controlled in compliance with the load, favourably a shoe calender.
The porosity of the paper that is aimed at and most appropriately manufactured by means of the method and the paper machine in accordance with the invention is higher than 150 Bendtsen units, measured in compliance with the Scan-P 60:87 standard, and the gloss is higher than 25 Hunter gloss units, measured in compliance with the Tappi 4/80 M-90 method. Characteristics of prior-art papers less well suited for multi-colour printing are, among other things:
basis weight: 80 . . . 200 g/m2 (grams per sq.meter), typically about 100 g/m2,
porosity: 150 . . . 1200 Bendtsen, typically 150 . . . 350 Bendtsen,
roughness: about 30 . . . 100 Bendtsen, typically about 50 Bendtsen,
thickness: 50 . . . 200 microns, typically about 100 microns,
gloss: 10 . . . 20 Hunter 75xc2x0,
non-coated, because coated papers are too dense, or
slightly (2 . . . 6 grams per sq.m per side) pigmented.
Thus, at present, no method is known for manufacture of a paper that is provided with the combination of porosity/gloss provided by the present invention.
In connection with the present invention, all conventional raw-materials of fine paper are suitable for use, both with short fibres and with long fibres, obtained from all wood species. As fillers, for example, carbonate and talc are used. Thus, the invention is carried into effect with existing paper raw-materials known in themselves.
When a paper in accordance with the invention is manufactured, in a way in itself known the additives, retention agents and fillers in the paper are applied as layers in the desired way from a multi-layer headbox to the wire part, by means of which procedure attempts are made to provide a chemical formation of layers in the Z-direction. When suitable additives and fillers are used, there is almost no need to act upon the distributions within the layers. Porosity of the paper can be produced in the wire part also by means of retention by using a retention agent and vacuums, whose use promotes the formation of porosity. However, it is also important that good formation is achieved. The former must be such that it does not damage the layer formation that has been produced. For example, a gap former is suitable for this purpose, but formers of other types are also suitable for use, at least to a limited extent. In view of improving the gloss, the desired distribution of fillers is substantially U-shaped. This form of distribution of fillers is obtained by means of said formation of layers in the paper. Thus, in this way, a better gloss is obtained after calendering.
In the press section, an extended-nip press is used, for example a shoe press. An extended nip is particularly well suited for this purpose, because in it the paper is compacted uniformly while the porosity is retained. Thus, in the Z-direction, a uniform distribution of density is obtained. If, for example, a roll press were used, the surface of the paper would be compacted to a higher extent than the middle.
In drying, favourably impingement drying is employed, which maintains the porosity of the paper, but traditional cylinder drying is also suitable for use.
In surface sizing and pigmenting, the film transfer method is used, in which case the paper remains more porous. In such a case, a uniform layer of coating agent is obtained, and the electric charge on the X/Y axis is even. If necessary, the web can also be pre-calendered before the film coating.
As the calender, a calender is used that substantially retains the porosity of the web preceding the calendering, preferably a shoe calender, in which case the porosity of the paper is retained and the gloss and the smoothness can be brought to the desired levels. As an extended-nip calender, it is also possible to use a so-called belt calender. With respect to a shoe calender and to a belt calender, reference is made to the applicant""s Patent Application FI 973954 and to the prior art referred to in said patent application. Also, for example, OptiLoad(trademark) calenders and soft calenders are suitable for the arrangement in accordance with the invention, but their use is not equally advantageous as the use of a shoe calender.
If necessary, it is possible to use pre-calendering before coating, in a way in itself known, in order to provide a low extent of pigmenting.
An important part in a paper machine in accordance with the invention is the headbox and the wire part, by whose means the formation of layers is produced, and a second important unit is the calender, by whose means the retaining of the porosity that has been achieved and the desired gloss are secured. The porosity and the gloss and the other properties of the paper grade to be produced are, of course, also affected by means of the surface sizing unit, the press section, and by means of the drying method. It has, however, been noticed that the formation of layers in the web and calendering of a correct type are the essential factors.